You might think of the eccentric old man next door as a dotard, but it wouldn't be very polite to refer to him that way out loud. A dotard is a person who's old and senile.
The noun dotard used to be a standard way to insult someone by implying that they were physically and mentally decrepit. Shakespeare, Chaucer, and J.R.R. Tolkien all used it regularly as a put-down between characters. Dotard is much less common these days. It comes from the Middle Low German doten, "be foolish," and when correctly pronounced, it rhymes with goatherd.